Lithium ion secondary batteries are being widely employed as a power source for driving electronic equipment. Negative electrodes for lithium ion secondary batteries whose active material is a graphite material have an average potential during the desorption of lithium ions of about 0.2 V (vs. Li/Li+) and exhibit a relatively flat potential. This potential is lower than that of the negative electrodes comprising hard carbon (non-graphitizable carbon). Therefore, equipment that requires high voltage and voltage flatness currently employs, as the power source, lithium ion secondary batteries comprising a negative electrode including a graphite material. Graphite materials, however, have a small capacity per unit weight of 372 mAh/g, and a further increase in capacity cannot be expected.
Meanwhile, materials capable of forming an alloy or compound with lithium are considered promising as the negative electrode materials which provide a high capacity. Such materials include silicon, tin, silicon oxide and tin oxide. During the absorption of lithium ions, however, the crystal structure of these materials changes so that the volume of the materials increases. For example, the composition of silicon in the state where the maximum amount of lithium ions are absorbed is represented by Li4.4Si. The volume of Li4.4Si equals 4.12 times the volume of Si. As for graphite in the state where the maximum amount of lithium ions are absorbed, its volume equals 1.2 times the volume of graphite containing no lithium.
A large volume change in negative electrode active material results in cracking of active material particles, insufficient contact between the active material and the current collector, etc. As a result, charge/discharge cycle life of the lithium ion secondary battery shortens. Particularly when cracking of active material particles occurs, the surface area of the active material particles increases. This accelerates the reaction between the active material particles and a non-aqueous electrolyte. Consequently, a film is formed on the surface of the active material. The formation of such film increases the interface resistance, which is considered as a major cause for short charge/discharge cycle life.
Under the circumstances, attempts have been made to form an amorphous silicon thin-film on a current collector having a rough surface so as to ensure space for relieving the expansion stress of active material as well as to ensure current collecting efficiency (see, e.g., Japanese Laid-Open Patent Publication No. 2002-83594). In order to increase the adhesion strength between the copper current collector and the amorphous silicon thin-film, this publication proposes to subject the amorphous silicon thin-film having formed on the current collector to heat treatment. By the heat treatment, a composite layer of silicon and copper is formed. Attempts have also been made to use a mixture of partially nitrided silicon oxide and a carbon material as a negative electrode active material (Japanese Laid-Open Patent Publication No. 2002-356314).
The negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2002-83594, however, has some problems. Because lithium ion conductivity in the silicon is low, polarization increases when charge/discharge is performed with a high current level, resulting in a low discharge capacity. Particularly when the thin film is composed only of silicon, a large concentration gradient of lithium is formed in the thickness direction, and the capacity tends to be low. Further, because silicon has an extremely large expansion coefficient, the resulting electrode is highly deformed. As a result, the electrode group in which the positive and negative electrodes are placed opposed to each other can be buckled, degrading the battery characteristics. Moreover, to relieve expansion stress at the interface between the silicon thin-film and the current collector, it is necessary to form silicon into a columnar shape, or to disperse copper in silicon by heat treatment, which requires enormous cost.
Likewise, the negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2002-356314 also suffers from some problems. Because the partially nitrided silicon oxide particle has low electron conductivity, a conductive material (e.g., carbon) needs to be added to the active material. As a result, the negative electrode has a low capacity density, so that battery capacity as expected cannot be obtained. In other words, an advantage of high capacity offered by the use of silicon cannot be obtained. Moreover, if graphite is used as a conductive material and propylene carbonate as a non-aqueous solvent for electrolyte, propylene carbonate decomposes on the surface of graphite during charge.